Quick answer
A half cylinder with the same radius and height holds exactly half the water of a full cylinder. The flat back panel is a diameter of the circle, so the footprint is a true semicircle.Shape differences
The full cylinder is round all the way around. The half cylinder is cut through its centre line, leaving a flat back and a semicircular front, a D shape from above.
Measurement differences
Both need radius and height. On a half cylinder the radius is half the width of the flat back panel.
Formula differences
Cylinder: V = π × r² × H. Half cylinder: V = π × r² × H ÷ 2.
Which calculator should you use?
If the tank is round all the way around, use the cylinder calculator. If it has one flat side and the depth from that side to the curve equals half its width, use the half cylinder calculator. If the depth is less than that, it is a bow front.
Example comparison
Cylinder, 12 in radius, 24 in tall
47.0 gal (177.9 L)
Half cylinder, 12 in radius, 24 in tall
23.5 gal (89.0 L)
Exactly half. Doubling the radius instead would quadruple either figure.